Frequency references or their production are known in various forms. One may mention as examples quartz oscillators, microwave transitions in atomic ground states (rubidium oven, cesium reference, hydrogen maser), and optical transitions in atoms (strontium) or ions (aluminum). For reasons of cost, mainly quartz oscillators are used in everyday applications, while atomic references are used where higher accuracies are necessary, for example in scientific, military, or communications fields. Important applications, besides the timekeeping itself, are navigation by means of satellite signals (for example GPS) or direct navigation by means of acceleration measurement, as well as network synchronization and frequency allocation for communications channels in radio, satellite, or cellular networks.
Quartz oscillators are cheap and compact and, through thermal stabilization, have time lags of less than 100 milliseconds per year and short-term stabilities of less than 1 picosecond per second. Cheap quartz references, as are used in commercial navigation systems, have accuracies in the range of 10−6, while military systems with precise temperature compensation have accuracies in the range of 10−8 to 10−10. This accuracy allows a more precise determination of location, since the position relative to the positioning satellites is expressed in units of time by means of the velocity of light. Atomic references are clearly more expensive, but they offer considerably higher accuracy. Cesium clocks are the international standard by which the second is defined, and thus are absolutely accurate by definition. However, they nevertheless have short-term variations, for example due to the eventual interaction time of the atoms with the oscillating field. Strontium clocks and aluminum ion clocks are the most accurate known references, but they require highly complicated laser systems and wells in order to cool the atoms and maintain them and to measure their reference frequency. Moreover, the reference frequencies provided thereby cannot be selected or varied, or can be selected or varied only to an insufficient extent. Further information on quartz references and atom-based frequency references are given in Reference 1 [“Quartz Crystal Resonators and Oscillators for Frequency Control and Timing Applications—A Tutorial”; J. R. Vig; Rev. 8.5.6.2; available at http.//www.ieee-uffc.org/frequency-control].
For some time now, optical resonators have also been used as frequency references, mainly to stabilize the optical frequency of a laser, as described in Reference 2 [“Laser-Based Measurements for Time and Frequency Domain Applications: A Handbook”; P. Maddaloni, M. Bellini, P. De Natale; Taylor & Francis (2013)]. One advantage of such references is the uninterrupted interaction of the light with the resonator. Disadvantages include susceptibility to vibration, frequency shifts due to changes of length, and frequency shift due to deposits on or alterations in the material of the mirror surface. In contrast to quartz-based references, optical references do not need moving parts. Unlike atom-based references, optical references also do not have intrinsic magnetic field dependency. Optical references are clearly cheaper than atomic references, but do not achieve their accuracy.
On the other hand, in view of the increasing transition from electronic methods to optical methods in telecommunications and data processing, it would be desirable to have highly accurate optical references that could be seamlessly integrated into modern data transmission and data processing structures.
Finally, in addition to said application problems, there is the fundamental theoretical problem that the current atomic time standards are dependent on physical parameters such as the fine structure constants. In this respect, a long-term stable, purely optical reference, which is determined only by geometry and the speed of light, would have a fundamental appeal, but currently, the necessary accuracy to be able to compete with atomic references cannot be achieved by optical references.